1. Field of the Invention
The present invention relates to a PWM (Pulse Width Modulation) motor drive circuit having a potential limiter. More particularly, the present invention relates to the potential limiter of the PWM modulation motor drive circuit connected between a drive IC member and a PWM converter circuit so as to steadily increase the motor speed to a predetermined speed during starting the motor.
2. Description of the Related Art
Referring to FIG. 1, a conventional PWM motor includes a PWM motor drive circuit 1 electrically connected with a motor coil 2 so as to carry out alternatively magnetizing the motor coil 2. The alternatively magnetized motor coil 2 can drive a motor rotor (not shown) to turn with respect to a motor stator of the motor. Typically, the PWM motor drive circuit 1 includes a drive IC member 10, a Hall IC member 11 and a PWM converter circuit 12. The drive IC member 10 electrically connects with the Hall IC member 11 such that the drive IC member 10 may receive rotor-detecting signals supplied from the Hall IC member 11. However, the drive IC member 10 is designed to have a pin VTH which electrically connects with the PWM converter circuit 12. Correspondingly, the PWM converter circuit 12 has a PWM input pin 121 serving to introduce a PWM signal from an exterior system (not shown). The PWM signal is converted into a voltage signal by the PWM converter circuit 12, and then sent it to the pin VTH of the drive IC member 10 for controlling or adjusting a period of alternative magnetization of the motor coil 2. Accordingly, operational statuses of the motor are possessed of multi-speed modes in heat-dissipating operation by means of the PWM signal.
Generally, the motor divides the operational statuses into a high-speed mode (including full speed), a low-speed mode (excluding full or zero speed) and a stop mode (zero speed). The drive IC member 10 can determine the operational statuses of the motor according to the input PWM signal such that the motor can be adjusted and changed in speeds to fulfill various system needs. For example, when a voltage of the pin VTH of the drive IC member 10 is higher than 3.0 V, the drive IC member 10 controls the motor to operate at the stop mode as well as zero rpm. Conversely, when the voltage of the pin VTH of the drive IC member 10 is lower than a predetermined voltage (e.g. 2.0 V), the drive IC member 10 controls the motor to operate at the high-speed mode as well as 6,000 rpm. If the voltage of the pin VTH Of the drive IC member 10 is in the range of 2.0 V to 3.0 V, the drive IC member 10 controls the motor to operate at the low-speed mode as well as 2,000 rpm.
Referring again to FIG. 1, the PWM motor drive circuit 1 is designed to have a capacitor 9 parallel-connected between the drive IC member 10 and the PWM converter circuit 12. Meanwhile, the capacitor 9 is designed to have a ground connection in place. In operation, the capacitor 9 is adapted to commutate a current input from the PWM converter circuit 12. However, the capacitor 9 of the PWM motor drive circuit 1 is so configured to stabilize the voltage of the pin VTH of the drive IC member 10. When the motor is actuated, the voltage of the pin VTH of the drive IC member 10 can determine and adjust the speed of the motor.
Referring to FIGS. 2A and 2B, the drive IC member 10 can control the motor to operate in the high-speed mode or the low-speed mode. In normal operation, the speed of the motor is operating at 2,000 rpm as well as low-speed mode when the voltage of the pin VTH of the drive IC member 10 is maintained at 3.0 V (i.e. lesser than 3.6 V but greater than 2.0 V). But, in abnormal operation, the speed of the motor is operating at the high-speed mode when the voltage of the pin VTH of the drive IC member 10 drops to zero volts (i.e. lesser than 2.0 V).
Still referring to FIGS. 1, 2A and 2B, an initial voltage across the capacitor 9 automatically drops to zero volts due to a ground connection and its transient short circuit, as best shown in FIG. 2A, and a voltage from a power supply can charge the capacitor 9 when the motor starts. Inevitably, the initial voltage of the pin VTH of the drive IC member 10 is maintained at substantially zero volts. In this way, the drive IC member 10 can invariably control the motor to operate in the high-speed mode as long as the motor starts; namely, the speed of the motor is designated to rapidly and shortly jump to 6,000 rpm (i.e. full speed) from zero rpm, as best shown in FIG. 2B.
Referring back to FIGS. 1 and 2B, once started, the motor must inevitably enter the high-speed mode that must rapidly and shortly increase the speed of the motor. However, there is no greater amount of operational heat for dissipation. However, ambient heat generated from a heat source is lower than a high temperature when the motor starts. This results in the motor unnecessarily operating at full speed (i.e. top speed) which generates an increased amount of air noise and vibration. Furthermore, the motor occurs an increased amount of abrasion among motor components which may shorten the longevity of the motor.
Referring again to FIGS. 2A and 2B, the voltage across the capacitor 9 can reach 3.0 V in the event after charging for a predetermined time. In this way, the voltage of the pin VTH of the drive IC member 10 is greater than 2.0 V but lesser than 3.6 V so that the drive IC member 10 terminates the motor to operate in the high-speed mode. Accordingly, the speed of the motor is designated to drop to a predetermined speed or a lower speed of 2,000 rpm. Therefore, it is undesirable to permit the drive IC member 10 to increase the speed of the motor reaching 6,000 rpm in the high-speed mode when it starts.
Referring to FIG. 3, to solve the problem discussed above, there is provided a connection of a capacitor 9′ in another PWM motor drive circuit 1 disclosed in applicant's own U.S. patent application Ser. No. 11/247,417, the entire disclosure of which is incorporated herein by reference. The capacitor 9′ connects between a power source and the pin VTH of the drive IC member 10. The power source is selected from a pin V6VREG of the drive IC member 10 which can build a predetermined voltage (i.e. 6 V) on the pin VTH of the drive IC member 10 to determine the mode of the speed of the motor when it starts. Consequently, once the motor starts, the initial voltage of the pin VTH Of the drive IC member 10 cannot drop to zero volts.
Referring now to FIG. 4A, the capacitor 9′ maintains the initial voltage of the pin VTH of the drive IC member 10 at 6.0 V due to electrically connection with the pin V6VREG of the drive IC member 10 and its transient short circuit when the motor starts. Subsequently, the voltage of the pin VTH of the drive IC member 10 can gradually drop to a voltage lower than 3.6 V and reach 3.0 V in the event after supplying the PWM signals to the motor for a predetermined time. Advantageously, the capacitor 9′ can avoid dropping the initial voltage of the pin VTH of the drive IC member 10 to nearly zero volts.
Referring now to FIG. 4B, there exists a delay period of time when the voltage of the pin VTH of the drive IC member 10 drops from 6.0 V to 3.6 V. In this delay period of time, the drive IC member 10 can control the motor situated in the stop mode. Advantageously, the motor can avoid entering the high-speed mode when it starts. Subsequently, the motor can enter the low-speed mode as well as 2,000 rpm when the voltage of the pin VTH of the drive IC member 10 is lower than 3.6 V.
Although the capacitor 9′ can be successful in eliminating the initial voltage of the pin VTH of the drive IC member 10 dropped to zero voltages, it can have some other disadvantages. The drive IC member 10 requires and controls the motor entering the stop mode and then changing to the low-speed mode. With regard to the problematic aspects forcibly entering the stop mode while starting the motor, the entire system is susceptible to causing a serious delay of time in control. In other words, it is undesirable to delay the operation of the motor or to completely stop the motor in the delay period of time. Hence, there is a need for improving the motor to prevent entering the stop mode while starting it.
To accomplish this task, the present invention intends to provide a PWM motor drive circuit having a potential limiter connected between a drive IC member and a PWM converter circuit. The potential limiter further electrically connects with a power source so that a voltage of a pin VTH of the drive IC member can be maintained at a predetermined voltage while starting the motor. Accordingly, the voltage changes of the pin VTH of the drive IC member can control the motor entering a low-speed mode and steadily increasing the speed while starting the motor in such a way as to mitigate and overcome the above problem.